fix: add darwin build constraints to Objective-C source files

Add //go:build darwin to secure_enclave.m and secure_enclave.h so Go
ignores them on non-darwin platforms. Without this, the lint stage fails
on Linux with 'Objective-C source files not allowed when not using cgo
or SWIG' because the !darwin stub (macse_stub.go) doesn't use CGO.

internal/macse/secure_enclave.m

@@ -1,3 +1,5 @@
+//go:build darwin
+
 #import <Foundation/Foundation.h>
 #import <Security/Security.h>
 #include "secure_enclave.h"